This invention relates to the production of an expanded mineral material suitable for use as a constituent or filler for concrete and other composite materials that contain a hydraulic or other type of binder and at least one particulate inorganic filler having a particle size in the range of typically from 0.1 to 20 mm.
Conventional concrete mixes consist essentially of cement, water, and a mineral filler which, depending upon its grain size, is termed "sand" or "gravel" and preferably consists of alluvial matter or "pebbles", i.e. particles of stone worn round and smooth by geo-formative forces and having a typical diameter of up to about 50 mm. Accordingly, both the sand as well as the gravel constituent of concrete may consist of "pebbles" and this latter term is used herein generically to indicate a generally round or spheroidal outer shape of individual mineral particles regardless of whether their diameters would be that of fine, medium or coarse sand (up to about 2 mm) or that of fine to medium gravel (above 2 mm). Particle sizes given herein are the maximum dimensions or "diameters" of generally spherical or spheroidal particles.
For producing improved construction materials including light-weight concrete it is known in the art to use bulky mineral "fillers" or additives such as expanded glass, expanded clay, or expanded mica. These conventional additives are obtained by heating a mineral composition, generally in the form of a stream of individual particles or pellets that contain a "latent blowing agent", i.e. a compound or constituent capable of producing gaseous matter, e.g. carbon dioxide or water vapor, when heated; the latent blowing agent may be incorporated into a preblend that is used to form precursor particles or pellets for subsequent thermal expansion, or may be a natural constituent, such as chemically bound water or a carbonate component of a mineral substance.
Specifically, expanded clay or expanded mica for use as a mineral filler is generally produced by passing a stream of particulated mineral material through a rotary kiln, that is, a cylindrical furnace revolving about an axis inclined at an angle of 5.degree. to 20.degree. towards the horizontal. The particles within the rotary kiln move or roll in contact with the furnace wall so that the outer layer or skin of the final particles will be rough and broken. Further, the structure of the internal voids will, in general, be "porous" in the sense that most voids, e.g. more than 50% communicate with each other; "cellular" structures, on the other hand, have fewer and typically less than 50% voids that communicate with one or more other voids.
A process for producing thermally expanded clay is disclosed in U.S. Pat. No. 2,676,892, for example, and use of vertical kilns or expansion reactors is known for producing expanded glass particles, e.g. as disclosed in U.S. Pat. No. 3,838,998 for making small hollow glass spheres, or in U.S. Pat. Nos. 2,978,339 and 3,030,215 for production of glass particles of which a desired majority (typically 70%) has a single void and an undesirable minority (typically 10%) has a porous structure while another undesirable portion (typically 20%) consists of unexpanded massive glass globules.
Prior art expanded mineral fillers suitable for concrete and the like have one or more of the following defects: the skin of the particles is rough and broken as a consequence of contact with the wall of the kiln during thermal expansion; the internal structure is porous rather than cellular in the sense defined above; mechanical or/and chemical stability under the conditions of use as a filler in concrete mixes is/are insufficient either because the expanded particles have an inherently low structural strength and/or low resistance against the environment typical in concrete mixes, or because the aqueous constituent of the mix will penetrate through the skin or its broken portions into the voids within the expanded particles; further, prior art production methods are costly and/or difficult to control.
Because of these defects, expanded materials obtained by prior art methods are far from ideal for use as a light-weight sand or gravel constituent of concrete or for other purposes where a generally stable and resistant low-cost light-weight mineral filler is desirable.
Such an improved light-weight material, also termed "expanded pebble material" herein, would be of use to replace, entirely or in part, sand and gravel constituents of concrete mixes, notably for construction purposes or for production of preformed construction elements, such as panels or blocks, and other uses where a substantially decreased specific weight and improved insulation (thermal and acoustic) of a structure or construction would be advantageous.
Now, according to the best knowledge of the inventor, no prior art method is capable to produce such a material and it is a main object of the present invention to provide for an improved process of producing an expanded mineral material suitable for the above described uses.
For production of small hollow glass spheres as disclosed in the above cited U.S. Pat. Nos. 2,978,339 and 2,978,340 and for production of hollow glass micro-spheres it is known to avoid contact between the fused glass and the oven by continuously dropping pellets of glass or glass-forming matter and a latent blowing agent into a vertical chamber in which a hot gas is moving upward and in counter-current to the pellets which are progressively heated to fuse and to expand.
According to these prior art methods, the expanded particles continue to move in a downward direction towards the lower end of the chamber where they are cooled and collected. The mass of the unexpanded pellets is critical because the amount of heat required for expanding the pellets to form the hollow globules increases geometrically with such mass. The extremely close process control required is prohibitive when production of a low-cost leight-weight filler for use as expanded pebbles in concrete is considered.
Further, and even more important, the sand and gravel constituent of a concrete mix must consist of granules or pebbles of differing diameters; if the sand and gravel constituent of a concrete mix is to consist substantially of expanded particles, such particles must be provided in relatively wide distribution of particle sizes, e.g. between 0.2 and 10 mm. So, if a prior art expansion method would be used wherein a substantially constant particle mass is critical because of heat transfer problems, simultaneous production of expanded particles of differing diameters would not be possible.